Written by Sarah Thompson - sarah@telergy.com

A simple Signals and Slots implementation for Arduino. Signals and slots is a handy pattern for situations where one or more objects need to be informed of changes occuring elsewhere in the program much like a callback. Unlike callbacks, with this library it's possible to have multiple functions called on the occurence of an event. For those of you who are unaware, boost::signals is a wonderfully useful signal/slot library which can be used alongside boost::bind for delegate-based event handling such as one sees in C#.

You can find the latest version of this documentation athttp://sigslot.sourceforge.net/

For downloads, mailing lists, bug reports, help and support, or anything similar, go tohttp://sourceforge.net/projects/sigslot

The author's personal web site is athttp://www.findatlantis.com/

Contents

Introduction

The C++ programming language is an amazingly capable beast. However, it is no panacea for all ills - it is just as possible to write spaghetti in this language as it is in any other (arguably easier than some).

C++ is usually described as an object-oriented language. Its claim to object orientation is generally appropriate, but unlike some other languages, C++ doesn't actually enforce good practice. Whether or not you think this is a good thing or a bad thing is up to you, I'm not interested in starting a religious war here, so I'll neatly sidestep the issue by changing the subject for a moment.

The Microsoft Foundation Classes (MFC) are a nightmarish mess. I'll say it again - the MFC is a programmer's nightmare. At one point it almost inspired me to give up programming and take up something less irritating, like becoming a professional 'nut the javelin' player or move to Outer Patagonia and become a cat wrangler. Nevertheless, the MFC is capable of supporting most things that you might want to do with the Windows platform, at the price of a little slice of one's sanity.

There are two reasons why the MFC is so painful. Firstly, it too-often seems to require an uphill struggle in order to get the simplest things to work. Secondly, it is very difficult and/or evilly messy trying to get disparate classes within an MFC application to talk to each other. The former problem is down to the design of the MFC itself - only deep experience using it can really get around this. The second problem is a consequence of the MFC being quite a thin wrapper around the underlying Windows SDK - getting one window to talk to another window often requires resorting to oldfashioned Windows 3.1 techniques like ::SendMessage() or ::PostMessage() with custom Windows messages. Yuck. That kind of code is difficult to write, difficult to debug, hard to maintain and generally speaking a Bad Idea.

Take a simple example. Imagine we want to create a dialog box, with a number displayed on it, and a couple of buttons which let you increment or decrement the number:

If you're going to program the MFC way, you'd slap in that dialog to the dialog editor in Visual Studio, then add message handlers for the 'Increment' and 'Decrement' buttons that use the MFC's DDX mechanism to update the number in the edit box. Easy enough you might think, but where's the reusability? I signed up to the card-carring object orientation movement on the basis that it was supposed to make it easier to reuse my code, or to use components written by other people with a minimum of pain. The MFC way of working might as well be Visual Basic, requiring cut-and-paste code reuse. Evil, nasty, bad.

What, in circumstances like this, I'd really like to be able to do is turn the numeric edit control and the increment/decrement buttons into genuinely reusable components in their own right. By component, I don't mean COM object or Active-X control, just a simple, lightwight, reusable C++ class.

The MFC way of doing this, reasonably enough, would be to inherit from the basic controls (CEdit and CButton in this example), then give those classes a common interface of some kind. You might, sensibly enough, add a couple of entry points to your numeric edit class, 'CMyEdit::Increment()' and 'CMyEdit::Decrement()', say, then in your CIncrementButton and CDecrementButton classes hold a pointer to a CMyEdit so that they can implement OnOK() handlers that call the relevant member functons in CMyEdit. This is fractionally better, but still not at all nice. What I'd like is to be able to create a general purpose button, that, er, well, 'clicks' when you click it, and an edit box that can be directed to do a variety of things remotely, such as increment, decrement, copy to clipboard, go back to zero, etc. The edit box shouldn't need to care what is calling it. Just as importantly, buttons shouldn't have to care what they are calling either, so the same, unmodified button class can be used to increment, decrement or clear the control this afternoon, and next week end up wired to my internet compliant remote garage door opener. Or whatever. But the important thing is that only the user of the classes should need to know (or care) how they are wired up - the classes themselves, to be genuinely reusable, should be above that kind of thing.

Speaking as an ex-hardware designer, I'd like software components to have pins around their edges, like ICs, so they can be 'wired up', but otherwise should be well behaved. To design a board with, say, an 8 bit microcontroller on it is made much easier by knowing that pin 34 is always Chip Enable (or something). I don't want to know how that pin is wired internally - I just want to know that it will work when I send a signal into it.

Credit where credit's due: Qt

The Qt library (see http://www.troll.no/ for further information) was the first attempt I personally ever saw at extending C++ by adding a 'signal-slot' metaphor to the language's existing repertoire of programming techniques.

Qt was a revelation to me when I started using it, which must be something like three years ago at the time of writing. For once, I could write code relatively quickly, with a reasonable chance of it actually working the way I intended it without the days of pushing water uphill with a fork that was usually required to breathe life into MFC applications. The thing that impressed me most about Qt was its signal/slot metaphor. Qt uses a preprocessor, moc, to preprocess an extended C++ syntax. Put briefly, any Qt class can possess one or more signals, and one or more slots. A slot is very much like an ordinary member function. Indeed, slots can be called directly as member functions, with the only syntactic difference being the need for the slotskeyword in the class header file. A signal in Qt is declared much like a member function, except that it has no local implementation - it just appears like a member function prototype in the header file. Signals can be connected, using the connect function, with any number of slots in any other C++ objects. When a signal is emitted, using the new keyword emit, all connected slots get called.

In principle, signals and slots are a bit like pointers, where a signal can be 'wired up' after the fact to the slots that need to be informed whenever it is emitted. Using pointers, be they function pointers, pointers to classes or pointers to class member functions, does have its own risks, of course. There are no guarantees, given such a pointer, that using it is safe - it is always necessary for the programmer to know, given the context of its use, that it is safe. Most programmers get this right nearly all of the time, of course. But when we get it wrong, our code goes horrendously bang, usually five minutes before a demo to the visiting CEO of your biggest client. I always tend to favour situations where the programming language picks up automatically on that kind of thing.

Signals and slots, in my opinion, have three major advantages over pointers:

Syntactically Neater. Signals & slots make for very readable code. You define your signals. You define your slots. You wire them up. That's it, and no surprises.

Inherently Robust. This is the neat part: when either end of a signal/slot connection is destroyed, the connection is automatically removed. It is therefore impossible to emit a signal and have it arrive at a class that has already been deleted. Best of all, you don't need to write any explicit cleanup code - if the caller goes out of scope, no problem. If the callee goes out of scope, no problem either.

Easier code reuse. Since the thing that has to type-agree is the signal and the slot, not the whole class interface, it is much easier to plug together disparate classes that had never initially been intended to work in that way. (Take it from me - I've done this in anger, and it really does make a difference)

I really like Qt. I think the people at TrollTech in Norway have done a brilliant job - their class library is far and away the best GUI library I've ever used (though the .NET framework actually comes remarkably close, but enough flame bait for one article).

More credit where more credit's due: Boost, GTK and James Slaughter

When I was working at Trayport in London, a coworker there, James Slaughter, got me interested in the Boost libraries, and was largely responsible for reawakening my interest in C++ as a programming language with real promise. Thank you James, if you're reading this. (As an aside, I was previously getting very interested in Objective CaML, but that's for another article at another time).

When I evangelised Qt within James' earshot, he opined that Qt was pretty good, but he wasn't keen on the moc preprocessor. Rightly enough, moc has a rather bad rep for mangling attempts to use templates in Qt-enabled classes, and it's always irritating to need to hack build scripts to preprocess code. James did mention that type safe signal/slot functionality was possible in C++. I'd kind of thought about it myself, but hadn't initially realised that it really was possible to do without something like moc.

I did a bit of a net search, and it seemed that a couple of people had put together C++ signal/slot libraries. One was in the pipeline for adoption by Boost (still is, at the time of writing). Another is due to the GTK bunch, as part of the GTK's C++ wrapper. Both were a little heavyweight for my needs (writing MFC code whilst staying at least a bit sane), and neither looked like good candidates for a straightforward Windows port.

I chose to write my own instead. The rest is history.

Downloading the sigslot library

You can download sigslot from the project downloads page.

Documentation

Detailed documentation for sigslot is currently in LaTeX format. I haven't yet decided whether to convert this to HTML and post it here, or to laboriously glomm it into the documentation system at SourceForge. For the moment, you can get the PDF version from the link below. If you want the LaTeX source, or are feeling kind and generous and would like to contribute by doing this conversion, please email me.

Reporting Bugs

So far, sigslot has been tested on VC++ 6.0 on Win32, VC++ 7.0 (Unmanaged) on Win32, gcc under Cygwin and Intel C++ on Win32. It has also been used in some real code without problems. However, it is relatively new, so tread carefully and please make sure to report any bugs, fixes or feature requests to the project team. By preference, please do this through the bug tracking system at SourceForge.

License

The sigslot library has been placed in the public domain. This means that you are free to use it however you like.

The author takes no responsibility or liability of any kind for any use that you may make of this library.

If you screw up, it's your fault.

If the library screws up, you got it for free, so you should have tested it better - it's still your responsibility.

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged(), and it has a slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue() slot:

The emit line emits the signal valueChanged() from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged() signal to the second object's setValue() slot using QObject::connect():

Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. Then b emits the same valueChanged() signal, but since no slot has been connected to b's valueChanged() signal, the signal is ignored.

Note that the setValue() function sets the value and emits the signal only if value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

A Real Example

The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect(). There are several ways to connect signal and slots. The first is to use function pointers:

There are several advantages to using QObject::connect() with function pointers. First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments. Arguments can also be implicitly converted by the compiler, if needed.

You can also connect to functors or C++11 lambdas:

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In both these cases, we provide this as context in the call to connect(). The context object provides information about in which thread the receiver should be executed. This is important, as providing the context ensures that the receiver is executed in the context thread.

The lambda will be disconnected when the sender or context is destroyed. You should take care that any objects used inside the functor are still alive when the signal is emitted.

The other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

All of these would work:

But this one won't work:

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...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

Lambda expressions are a convenient way to pass custom arguments to a slot:

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

Ucl Library Slots

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

Kcl Library Slots

See also QLCDNumber, QObject::connect(), Digital Clock Example, Tetrix Example, Meta-Object System, and Qt's Property System.

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